1. Field of the Invention
This invention relates to telephone supervisory control circuits and, more particularly, to ring trip control circuits.
2. Description of the Prior Art
It has become increasingly common to utilize carrier-derived subscriber channels on a single pair of metallic telephone wires to postpone, or obviate the necessity for, the installation of expensive telephone cable. One such carrier system is disclosed in the copending application of the present applicant, Ser. No. 974,384, filed of even date herewith.
One of the problems associated with such carrier systems is that direct current and low frequency signals can no longer be used for supervisory signaling on the metallic loop. It is therefore necessary to provide an alternative method of supervisory signaling for each of the carrier-derived channels in such a system.
In order to provide ringing signals in a carrier system such as that described above, it is often necessary to generate the ringing signals locally at the remote and of the carrier system and to control the application of such ringing signals to the telephone subscriber's station set by supervisory signals transmitted on the carriers themselves. In the aforementioned copending application of the present applicant, such ringing is provided by gating a high voltage source onto the local subscriber's drop wires at a ringing signal rate, e.g., 20 Hz.
In such a system, as in all other application of ringing signals, it is necessary to detect when the subscriber goes off-hook (i.e., removes his handset from the telephone base) so as to terminate the ringing signals. This function, called "ring trip," is normally provided by relays at the central office which respond to the talking current which begins to flow in the subscriber loop in response to the off-hook condition. In a carrier-derived channel, such ring trip detection is no longer possible and means must be provided to accomplish this function at the remote end of the carrier facility.
It is also necessary to provide ring trip detection separately for each of the carrier-derived channels supported by the single transmission facility. Since these circuits must be located at the remote end of the carrier facility, it is essential that they consume little power and be small in size.